This invention relates generally to cable antenna television (CATV) systems and particularly to a cable signal decoder for such a system that may be used with both cable television systems having in-band data and cable systems having out-of-band data.
Cable systems are generally categorized into in-band and out-of-band data types. The in-band type generally sends data in the vertical blanking interval of the television signal, by modulation of the horizontal sync interval, or via the audio subcarrier of the channel signal. Out-of-band systems utilize a separate carrier signal that is outside of the television frequency spectrum to send data. The data is usually sent in the form of a frequency shift keyed (FSK) carrier. The advantage of an out-of-band system is that a cable system decoder unit may be addressed at any time, irrespective of the channel to which it is tuned. The disadvantage is the added cost of the special receiver for recovering the out-of-band data and the low level of security against "trapping out" addressable data. The advantage of an in-band data system is its economy and higher level of protection for data. The disadvantage is that the subscriber decoder unit must be tuned to an in-band data channel before the cable head-end can communicate with the decoder unit. Both forms of data transmissions are commonly used in cable television systems. Accordingly, it would be advantageous to provide a CATV receiver that has the capability of operating with cable systems having in-band data as well as with systems having out-of-band data to enhance service flexibility and minimize obsolescence. It would also be advantageous to have a dual system where certain data would be sent in-band for security and other data sent out-of-band for communication efficiency.